1. Field of the Invention
The invention relates to signal processing circuit, and more particularly to a single-ended input to differential-ended output low noise amplifier circuit.
2. Description of the Related Art
A single-ended antenna is a widely used type of antenna. An image-rejection mixer for signal down-conversion is however implemented by a differential-ended input double-balanced mixer. The single-ended input signal received by the single-ended antenna has to be converted into differential-ended output signal. Therefore, the signal received by the antenna can be post-processed after down-conversion through the image-rejection mixer.
There are two conventional methods for converting the single-ended input signal into the differential-ended output signal. One is a single-ended input to differential-ended output amplifier circuit 100 presented in FIG. 1, comprising a balun 12 and a differential-ended input to differential-ended output amplifier 13. Differential-ended input to differential-ended output amplifier 13 can be a low noise amplifier (LNA). The method is implemented by adding balun 12 outside the chip. Balun 12 has two inputs, one is coupled to an antenna 11, and the other is coupled to the ground. Two outputs of balun 12 are respectively coupled to two outputs of differential-ended input to differential-ended output amplifier 13. Balun 12 is composed of capacitors and inductors. One input of balun 12 receives the single-ended input signal from antenna 11 and the single-ended input signal is converted into the differential-ended output signal. The single-ended input signal can be a radio frequency (RF) signal. Then, the differential-ended output signal transferred to differential-ended input to differential-ended output amplifier 13. Differential-ended input to differential-ended output amplifier 13 amplifies the differential-ended output signal and generates a differential-ended output amplified signal. Accordingly, the differential-ended output amplified signal of the single-ended input to differential-ended output amplifier circuit 100 can be transferred to the two inputs of an image-rejection mixer 14 for down-conversion.
Typically, differential-ended input to differential-ended output amplifier 13 and image-rejection mixer 14 are designated inside the chip. Since the signal received by antenna 11 has to be post-processed through image-rejection mixer 14 after down-conversion, the method of single-ended input to differential-ended output amplifier circuit 100 shown in FIG. 1 is adopted. In FIG. 1, balun 12 is added outside the chip. Thus, image-rejection mixer 14 can receive the differential-ended output amplified signal and which can then be post-processed. Because the method requires additional components, the cost of the chip module may be increased.
FIG. 2 is another single-ended input to differential-ended output amplifier circuit 200, comprising a single-ended input to single-ended output amplifier 22 and a single-ended input to differential-ended output inductive transformer 23. Single-ended input to single-ended output amplifier 22 can be a low noise amplifier. The method is implemented by adding a single-ended input to differential-ended output inductive transformer 23 to the chip comprising single-ended input to single-ended output amplifier 22 and an image-rejection mixer 24. An antenna 21 is coupled to single-ended input to single-ended output amplifier 22. The output end of single-ended input to single-ended output amplifier 22 is coupled to the input of single-ended input to differential-ended output inductive transformer 23. Antenna 21 receives single-ended input signal which is then transferred to single-ended input to single-ended output amplifier 22. The single-ended signal can be a radio frequency signal. Single-ended input to single-ended output amplifier 22 amplifies the single-ended signal into an amplified signal, which is then transferred to single-ended input to differential-ended output inductive transformer 23. Single-ended input to differential-ended output inductive transformer 23 amplifies the amplified signal into differential-ended output signal. Thus, the differential-ended output signal of single-ended input to differential-ended output amplifier circuit 200 can be transferred to two inputs of image-rejection mixer 24.
However, single-ended input to differential-ended output amplifier circuit 200 shown in FIG. 2 is implemented by adding single-ended input to differential-ended output inductive transformer 23 to the chip. Single-ended input to differential-ended output inductive transformer 23 composed of capacitors and inductors need a large die area, although the cost for additional components is reduced.
Presently, a differential amplifier pair with grounded negative terminal can be used to replace single-ended input to differential-ended output inductive transformer 23 shown in FIG. 2. FIG. 3A is a conventional single-ended input to differential-ended output amplifier circuit 300. An antenna 31 is coupled to a single-ended input to single-ended output amplifier 32. Single-ended input to single-ended output amplifier 32 can be a low noise amplifier. An output amplified signal of single-ended input to single-ended output amplifier 32 is coupled to an input of a differential amplifier pair circuit with grounded negative terminal 33.
FIG. 3B is differential amplifier pair circuit with grounded negative terminal 33 comprising a first transistor 301, a second transistor 302, a first resistor 303, a second resistor 304, a current source 305, a capacitor 306, a first output 307, a second output 307′, a voltage source VDD, a ground VSS and a first node 3A. The gate of first transistor 301 is coupled to the output of single-ended input to single-ended output amplifier 32. The drain of first transistor 301 is coupled to first output 307. The source of first transistor 301 is coupled to first node 3A. The gate of second transistor 302 is coupled to capacitor 306. The drain of second transistor 302 is coupled to second output 307′. The source of second transistor 302 is coupled to first node 3A. First resistor 303 is coupled between first output 307 and voltage source VDD. Second resistor 304 is coupled between second output 307′ and voltage source VDD. Current source 305 is coupled between first node 3A and ground VSS. Capacitor 306 is coupled between second transistor 302 and ground VSS. Therefore, the differential-ended output signal of single-ended input to differential-ended output amplifier circuit 300 can be transferred to two inputs of a image-rejection mixer 34 through first output 307 and second output 307′ for down-conversion.
However, conventional differential amplifier pair circuit with grounded negative terminal 33 is used to replace single-ended input to differential-ended output inductive transformer 23 to reduce die size. The quality of the differential-ended output signal at the output of single-ended input to differential-ended output amplifier circuit 300 may be influenced by noise generated by the chip, which may be coupled to the input of differential amplifier pair circuit with grounded negative terminal 33 through the bypass capacitor 306 thereof.